It means that your Renegade client sent a team preference while teams are automatically assigned (and thus, you are not allowed to pick a team). This should normally not be possible. Was the server modified? Perhaps try disabling your plugins temporarily, and see if any of those causes this.
As a workaround if this keeps happening, you can try the following. Does the team selection dialog show up for you when you attempt to join? If so, select "auto" instead of Nod/GDI. If not, start Renegade, go to "Multiplay Internet", then "Westwood Online", then in the menu on the left, choose "My Information" and make sure that "Preferred Side" is set to "Random". Let me know if any of this helps.